The Structure Core will support all three projects in this U19. The goal of our U19 research program is to use structure-based rational design to combat influenza virus from both the therapeutic antibody and vaccine perspective. These endeavors, targeting the receptor-binding site (RBS) on hemagglutinin (HA), require many rounds of design and experimental validation executed in an iterative manner. Thus, the focus of this Core is three-fold: (i) Naturally occurring antibodies from Project 1 will be studied in complex with natural HA molecules. (ii) to solve structures of HA in complex with engineered antibodies from Project 2 for therapeutic applications, and (iii) to solve structures of HA engineered scaffolds in complex with known broadly neutralizing antibodies (bnAbs) in Project 3 for vaccine applications. We will use both electron microscopy (EM) and x-ray crystallography to accomplish these aims and generate sufficient structural coverage to enable development of anti-influenza solutions. Specifically, we will use electron microscopy (EM) to solve structures of HA in complex with engineered antibodies. EM is uniquely suited to rapidly generate structural information of protein complexes in a high-throughput manner. The information from EM can directly inform further structural and design efforts. We will employ both negative stain and cryo-EM methods to evaluate antibody-HA complexes at moderate and high resolution, respectively. We will also use x-ray crystallography to solve structures of HA and engineered HA scaffolds with antibodies. X-ray crystallography is routinely used to reveal atomic-level details of proteins and their complexes. Using the crystal screening capacity of The Joint Center for Structural Genomics housed at Scripps (Dr. Wilson is the PI), we can screen large numbers of antibodies, antigens, and conjugate complexes. We will use this pipeline to solve atomic-resolution structures of antibody-HA and antibody-scaffold complexes.